Powertrain comprising an optimized energy recovery system

ABSTRACT

A powertrain includes a variable ratio transmission device having an input shaft coupled to an engine and an output shaft coupled to a driven unit, a three-way power split transmission device including three input/output couplings, a first of which is coupled to a flywheel and a second of which is coupled to an electrical machine. The third input/output coupling of the three way power split transmission device is mechanically coupled to the engine and to the input shaft of the variable ratio transmission device.

BACKGROUND AND SUMMARY

The invention relates to a powertrain equipped with an optimized energyrecovery system comprising a flywheel.

The reduction of the fuel consumption is a major stake for thesustainability of many industries, but most important for the automotiveindustry and the machinery industry. A huge majority of vehicles(trucks, buses, passenger cars, etc.) is fitted with a powertraincomprising an internal combustion engine which drives the driving wheelsthrough a transmission set (including for example a clutch or a torqueconverter, a manual, automated or automatic gearbox, a differential, anaxle) and on another part the auxiliaries that are necessary to operatethe vehicle systems. Some of these auxiliaries are fed by an electricalnetwork, which energy comes from a generator run by the ICE. In the caseof commercial vehicles, the engine is most often a turbo-charged dieselengine. Similar powertrains are used to power all sorts of machines,including construction equipment machines.

When analyzing the balance of energy used to operate a vehicle, there isa non reducible energy demand due to the drag and rolling forces thatare intrinsic characteristics of the vehicle. This one being put apart,an amount of the energy which is used to drive the vehicle is wasted inthe brakes, coming from potential energy kinetic energy (resulting fromthe energy provided to the vehicle when accelerating), and potentialenergy (resulting from energy provided to the vehicle during hillclimbing).

Some well known technologies can limit a fraction of these losses. Forexample, hybrid drivelines are a known technology to achieve brakingenergy recovery.

A first of these technologies comprises hybrid powertrains which equipthe now well-known Hybrid Electric Vehicles (HEVs) with electro-chemicalstorages. On a general level, HEVs comprise a powertrain associating anICE with at least one electric machine and with at least one storagedevice (batteries, super capacities, inertia wheels . . . ). Such asystem can store an amount of the braking energy in the storage deviceby using the electric machine as a generator and then, at an appropriatetime, redirect this energy to the driveline using the electric machineas a motor to participate to the propulsion of the vehicle.

One interesting layout for a hybrid electric vehicle is the so-calledparallel hybrid layout where the transmission set of the vehicle ismechanically coupled (directly or through gearings, belts) in parallelto both the ICE crankshaft and to the electrical machine. Duringdecelerations, the electric machine is used as a generator to slow downthe vehicle and the electricity that is produced is stored in batteriesor in ultra-capacitors. During accelerations, the electric machine isused as a motor and adds its power to that of the ICE, or even replacesthe power of the ICE.

Hybrid powertrains are also known in the field of construction equipmentmachinery.

As a variant, it is also know to have hybrid powertrains withelectro-mechanical storage means. Instead or in addition of batteries,an electro-mechanical device can be used where the energy is stored inkinetic form, for example in a spinning wheel also called a flywheel. Inthis context, the flywheel is a dedicated energy accumulating flywheelwhich is not to be confused with the ICE flywheel, the sole purpose ofwhich is to smoothen the rotation of the engine. An energy accumulatingflywheel needs to store a significant amount of energy which can beenough to drive the vehicle, at least as a complement to the ICE.

In a known layout, the flywheel is mechanically linked to a firstelectric machine, the purpose of which is to speed up or slow down theflywheel in order to increase or decrease its kinetic energy andtransform it into electricity. In a parallel HEV, the electric energyderived from the flywheel can then be used in a second electricalmachine to drive the vehicle. The second electrical machine is also usedas a generator, during deceleration, to provide the first machine withthe electricity to drive the flywheel.

Compared to electrochemical batteries, flywheels are likely to be aneconomic alternative when:

-   -   rapid discharge and recharge are necessary;    -   long life cycle is necessary;    -   weight or volume is constrained;    -   use of toxic or explosive materials is unfeasible; and/or    -   environmental control is difficult.

Compared to ultracapacitors or batteries, flywheels are likely to be theeconomic alternative when industrial voltages must be supported.

When high energy capacities are needed, flywheels must rotate at highspeeds with safety issues (risk of burst of the flywheel due to inertiaforces) and technological issues (such as issues with the bearing). Inthe above described layout of a hybrid powertrain equipped with aflywheel, the two electrical machines need to be rated to the maximalpower to be recovered. Moreover, in such a layout, all the energyrecovered by the system has to be first transformed from mechanical formto electrical form, then from electrical form to mechanical form to bestored in the flywheel, and again from mechanical form to electricalform, then from electrical form to mechanical form to be used as drivingpower. Although electric machines have inherently good efficiencyratios, these numerous transformations necessarily lead to energylosses.

Document US-2007/0049443 discloses another type of layout for a vehicleequipped with an energy recovery system. The layout is based on aconventional ICE driveline with an ICE, a torque converter with a lockclutch, an automatic gearbox and a transmission shaft which is coupledto the final drive. On this base, an energy accumulating flywheel iscoupled to the output shaft of the gearbox through a three-way powersplit transmission device comprising three input/output couplings, afirst of which is coupled to the flywheel, a second of which is coupledto a first electrical machine and the third of which is coupled to thetransmission shaft of the gearbox. The three way power splittransmission is embodied as a planetary gear. The system furthercomprises a second electrical machine which is directly coupled with theengine output shaft. Essentially, the first electrical machine and theplanetary gear form a continuously variable transmission between theflywheel and the transmission shaft, said transmission beingelectrically controlled by the first electrical machine.

Compared to the above described conventional layout, the layout ofdocument US-2007/0049443 is more favorable because part of the energyrecovered through the flywheel is transferred purely mechanically, thatis with a high efficiency ratio. Nevertheless, this layout has somedrawbacks. As can be seen from the graph of FIG. 2 in documentUS-2007/0049443, during acceleration or deceleration of the vehicle, thespeed of the flywheel (which corresponds to its “state of charge”) isheavily dependant on the vehicle speed. To keep the flywheel at acertain speed, or to bring it to that speed, the first electricalmachine is constantly operated within an extended range of speeds. Onlyfor one defined vehicle speed is the first electrical machine at rest,at which moment the recovered energy is transferred entirelymechanically. For whatever other speed, part of the energy istransferred electrically, and the more the vehicle speed is differentfrom the defined speed, the more important the electrical part. Thislayout, with the three way power split transmission on the transmissionoutput shaft, also leads to relatively big torque values beingtransferred trough the recovery system, which therefore need bedimensioned consequently.

Therefore, it is desirable to provide a new layout for powertrainequipped with an optimized energy recovery system comprising a flywheel,so that the size of the system can be reduced, so that it can be betterintegrated on a vehicle or a machine, and so that it can have a betterefficiency ratio.

An aspect of the present invention provides for a powertrain comprising:

-   -   a variable ratio transmission device having an input shaft        coupled to an engine and an output shaft coupled to a driven        unit    -   a three-way power split transmission device comprising three        input/output couplings, a first of which is coupled to a        flywheel and a second of which is coupled to an electrical        machine, characterized in that the third input/output coupling        of the three way power split transmission device is mechanically        coupled to the engine and to the input shaft of the variable        ratio transmission device.

DESCRIPTION OF FIGURES

The invention will be best understood from the following detaileddescription which refers to the appended drawings in which:

FIG. 1 is a schematic functional diagram of a first embodiment of theinvention;

FIGS. 2 to 4 are figures similar to FIG. 1, showing respectively asecond, a third and a fourth embodiment of the invention;

FIG. 5 and FIG. 6 are each a schematic view of respectively a first anda second possible structural implementation of the first embodiment ofFIG. 1;

FIG. 7 and FIG. 8 are each a schematic view of respectively a first anda second possible structural implementation of the second embodiment ofFIG. 2;

FIG. 9 is a schematic view of a possible structural implementation ofthe third embodiment of FIG. 3;

FIG. 10 is speed diagram showing the speed relationship which links thethree input/output couplings in a planetary gear

FIGS. 11 a and 11 b are respectively a speed diagram and a functionaldiagram showing the respective speeds and power flows for a specific useconfiguration in which the flywheel can be charged;

FIGS. 12 and 12 b are respectively a speed diagram and a functionaldiagram showing the respective speeds and power flows for another useconfiguration in which the flywheel can be charged;

FIGS. 13 a and 13 b are respectively a speed diagram and a functionaldiagram showing the respective speeds and power flows for a specific useconfiguration in which the flywheel can be discharged;

FIGS. 14 a and 14 b are respectively a speed diagram and a functionaldiagram showing the respective speeds and power flows for another useconfiguration in which the flywheel can be discharged;

FIGS. 15 and 16 are each a schematic view of further possible structuralimplementations of the second embodiment of FIG. 2; and

FIG. 17 is a schematic view of further possible structuralimplementations of the third embodiment of FIG. 3.

DETAILED DESCRIPTION

On FIG. 1 are shown the main elements of a powertrain 10 according tothe invention, this powertrain being incorporated in a vehicle for itspropulsion.

Nevertheless, it is to be noted that the powertrain according to theinvention could also be used in other contexts, such as in the field ofconstruction equipment machinery, including loaders, excavators, etc. Insuch a case, the driven unit would not be (or not only be) a vehicleaxle, but could also be a hydraulic pump for a hydraulic power circuit.

The powertrain 10 comprises first of all an engine 12. Although theinvention could be implemented within a powertrain where the engine isan electrical engine, it will be hereunder more specifically consideredthe case where the engine is an internal combustion engine. Forcommercial vehicles, especially trucks and buses, the engine can be aturbocharged diesel piston engine. The engine 12 has an output shaft 14which is coupled to the input shaft 16 of a variable ratio transmissiondevice 18 which, in the example shown, is a conventional discrete ratiomechanical gearbox. This mechanical gearbox could be automated, that iswhere the gear selection is not done manually but through actuators. Thegearbox could also be a discrete ratio automatic gearbox. In specialcases, one could implement the variable ratio transmission device as acontinuously variable transmission (CVT).

In the example shown, the engine output shaft 14 is coupled to thegearbox input shaft 16 through a first clutch 20 and a second clutch 22,the two clutches being coupled by an intermediate shaft 24. In certaincases, such as if the gearbox has a neutral position, one of these twoclutches could be dispensed with. In such a case, the intermediate shaft24 will simply be unitary with either the engine output shaft 14 or thegearbox input shaft 16, depending on which clutch remains. In the caseof an automatic gearbox, at least one of the clutches could be replacedby a torque converter.

The gearbox 18 has an output shaft which is coupled to a driven unit 25comprising for example a propeller shaft 26, a differential 28, and twodrive shafts 30 driving the two wheels 32 of a drive axle. The drivenunit could be different, depending on the vehicle, for example withseveral driven axles.

The above mentioned components, including the engine 12, the clutch(es)20, 22, the gearbox 18 and the driven unit 25 can be considered asforming the main driveline of the powertrain according to the invention.

According to one feature of the invention, the powertrain 10 is equippedwith an energy recovery system which includes a three-way power splittransmission device 34 comprising three input/output couplings. Aconventional differential is such a type of device, but, in the exampleshown, this device is implemented as a planetary gear 34. Such a gearcomprises a sun wheel 37, a ring wheel 41 coaxial with the sun gear 41,and a carrier 43 which is coaxial with the sun and ring wheels, wherethe sun wheel, the ring wheel and the carrier are rotatable one to theother around their common axis, and where said carrier 43 carriesrotatable satellite wheels 45 which are meshed with both the sun wheel37 and the ring wheel 41. Each of the sun wheel, of the ring wheel andof the satellite carrier can be considered as one input/output of theplanetary gear. In such a gear, the speed of the three inputs/outputsare linked one to the other in way which will be explained further.

According to one embodiment of the invention, a first input/outputcoupling 36 of the three way power split transmission device 34 ismechanically coupled to a flywheel 38, a second input/output 40 ismechanically coupled to a first electrical machine 42, and a thirdinput/output 44 is mechanically coupled to the engine 12 and to theinput shaft of the gearbox 18. The energy recovery system can be said toinclude essentially the planetary gear 34, the flywheel 38 and the firstelectrical machine 42.

It is to be noted that the invention can be implemented with whicheverof the sun wheel, the ring wheel or the planet carrier being the first,second or third input/output of the power split device. Nevertheless, atleast for a commercial vehicle powertrain, a favorable layout can beachieved with the flywheel coupled to the sun wheel of the planetarygear, with the engine coupled to the satellite carrier and with thefirst electrical machine coupled to the ring wheel.

As can be seen on FIG. 1, the power split device 34 is coupled to theintermediate shaft 24 which extends between the two clutches. Therefore,when the first clutch 20 is engaged, the power split device is directlycoupled (through a transmission device) to the engine output shaft 14,and, when the second clutch 22 is engaged, it is directly coupled to thegearbox input shaft 16. In other words, if one considers the generalcase of power flowing downstream along the main driveline from theengine 10 to the vehicle wheels 32, the power split device 34 ismechanically coupled to that main driveline upstream of the variableratio transmission device 18.

As said above, the flywheel 38 is an energy accumulating flywheel whichis not to be confused with the engine flywheel. Indeed, at least in thecase of an internal combustion piston engine, the engine comprises itsown conventional flywheel (not shown on the figures) which is dedicatedto achieve a smooth engine rotation, despite of the inherentlydiscontinuous operation of a piston engine. Calculations have shownthat, for a powertrain dedicated to a medium size truck, it would bedesirable for the energy accumulating flywheel to be able to storeenergy in the order of 100 to 300 Watt×hours. Further calculations haveshown that this could be achieved with a steel disc having a diameter ofless than 500 mm, weighing approximately 50 to 150 kilograms, and beingrotatable up to a speed of 4500 to 9000 rpms. Such a flywheel would havean axial thickness approximately within a range of 30 to 100 mm and amoment of inertia approximately within a range of 1500 to 5000 kilogram,m2. In any case, the energy accumulating flywheel 38 can bedifferentiated from the engine flywheel by the fact that its rotatingspeed is not a linear function of the rotating speed of the engine 12.

The first electrical machine is electrically connected to a powercontrol unit 48, the aim of which is to manage the electrical energy inthe electric machine 42. The power control unit 48 is electricallyconnected to an electric network 50 which has a storage unit 52 (in theform of batteries, super capacities, etc.), and passive electricalauxiliaries 54 (typically vehicle electrical systems that only consumeelectrical energy from the power unit). The electric network 50 may alsocomprise active electrical loads 56, for example electric machines whichare directly or indirectly mechanically coupled to one of theinputs/outputs of the power split device 34. The electric network canalso comprise a resistor 53 (which can be equipped with a coolingsystem) to dissipate excess electric energy produced by the firstelectrical machine in certain operating modes where the electric storageunit 52 is full and where the other electric consumers cannot use allthe produced electricity.

One or several electronic controller unit(s) (not shown) manage(s) allthe components in order to make them operate properly as wanted.

The first electrical machine 42 can operate both as a motor or as agenerator, and in its both rotating directions. The electrical machine42 will be able to provide the second input/output 40 of the power splitdevice 24 a torque, either resistive or driving, in both directions.Therefore it will either draw electric current from the electricalnetwork or provide electrical current to the network. Calculations haveshown that a suitable electrical machine could have a power rating of 20to 80 kWh.

As can be seen on FIG. 1, a brake 46 can be provided to act on thesecond input/output 40 of the power split device 34, together with thefirst electrical machine 42. Such a brake can be of any type. It will beused to complement or replace the first electrical machine in some caseswhen the machine is used to provide a resistive torque to the powersplit unit. With such a brake, a smaller first electrical machine 42 canbe used, while still yielding substantially the same beneficial resultsof the invention. In the depicted example, the brake is mounted inparallel to the first electric machine, for example through a gear trainadapted to drive the brake in its best operating speed range. The brakecould also be mounted in series simply on the shaft between the powersplit device and the first electrical machine if the brake and the firstelectric machine have the same operating speed range.

Each of the flywheel 38, the first electrical machine 42 and the engine12 can be coupled to the corresponding input/output of the power splitdevice 34 through a transmission, for example through two meshed wheelsforming a reduction gear. As will be seen hereunder, a transmissiondevice could be a further planetary gear used as a reduction gear, abelt and pulley transmission, etc Preferably, these transmission devicesare mechanical devices giving a constant ratio of angular speeds betweeninput and output. These transmissions can have a clutch to decoupleinput and output.

In this first embodiment of the invention shown on FIG. 1, thepowertrain 10 has no active loads. As will be seen, all the energy whichis used to run the first 42 electric machine comes from the electricalstorage 52. The powertrain 10 must thus be tuned in order to reduce therating of the electrical storage (in terms of capacity and ofcharging/discharging power) through a proper choice of thecharacteristics of the transmission and of the planetary gear 34.

The powertrain 10 according to the invention has different operatingmodes, some of which are described below, with reference to FIGS. 10 to14 b

FIGS. 10, 11 a, 12 a, 13 a, 14 a are based on the conventional speeddiagram of a planetary gear. In the example shown, it is recalled thatthe sun wheel 37 is coupled to the flywheel 38, the ring wheel 41 iscoupled to the first electrical machine 42 and the carrier 43 is coupledto the engine 12. On such a speed diagram the three vertical axis S, C,R represent respectively the rotating speed axis of the sun wheel 37, ofthe carrier 43 and of the ring 41 wheel with respect to the value 0represented by the horizontal axis. Of course, the speed can be positiveor negative depending on the rotation direction of each of thesecomponents. Especially, the ring wheel and the sun gear can rotate inthe same direction or in opposite directions depending on the rotationspeed of the carrier. In such a diagram, the axis S, C, R are spacedapart so that the ratio of distance between axis S and C to the distancebetween axis C and R is proportional the ratio of the diameter of thering wheel to the diameter of the sun wheel. In such a case, therotation speeds of the three components are interrelated so that theirrepresentative locations on their respective axis are aligned. In otherwords, with such a diagram, it is possible to know the speed ofcomponent by knowing the speed of the other two components.

On FIG. 10 is shown the case where, from an initial state of operation,shown as a solid line, no torque is provided by the first electricalmachine 42. It is recalled that, in a planetary gear 34, the torquesapplied on the three inputs/outputs are by nature balanced as on alever.

In such a case, any increase in the carrier speed (i.e. the enginespeed, and consequently the vehicle speed) will simply result in anincrease of the electrical machine speed which is coupled to the ringwheel, the flywheel (coupled to the sun wheel) keeping its previousspeed. Therefore no power is transferred to or from the electricalnetwork 50 and no power is transferred to or from the flywheel 38. Notorque is transmitted through the power split device 34, and the engine12 provides all the energy to drive the vehicle transmission.

On FIGS. 11 a and 12 a are shown two cases where regenerative braking isoccurring. This is shown on the simplified functional diagrams of FIG. 1ib and 12 b (which correspond respectively to the speed diagrams ofFIGS. 11 a and 12 a) where it is represented by an arrow that the maindriveline is able to contribute power to the power split device 34. Thispower decelerates the vehicle without the use of the vehicle servicebrakes. We will use the convention according to which the torque appliedby the main driveline to the power split device 34 in such a case ispositive, represented by an upward arrow on the speed diagram. On FIG. 1lb is represented the corresponding power flux in the system. In such acase, the first electrical machine 42 can be controlled in order toforce the power split device 34 to send some power through its thirdinput/output to the flywheel 38 so as to increase its rotating speed,thereby transforming this transferred power into energy stored inmechanical form. Therefore, as can be seen on FIG. 11 a, the firstelectrical machine 42 can be controlled to exert a certain torque on thepower split device 34. Depending on the rotation direction of theelectrical machine 42 (opposite cases are shown on FIGS. 11 a, 12 a andFIGS. 1 ib, 12 b), the electrical machine 42 will be controlled eitheras generator (whereby it extracts mechanical power from the power splitdevice 34, as shown on FIG. 1 ib, this power being transformed intoelectricity and provided to the electrical network 50), or as a motor(whereby it provides mechanical power to the power split device 34, asshown on FIG. 12 b, this power being provided by the electrical network50) to, in both cases, apply on the power split device a torque which,according to the above convention, is negative (see FIGS. 11 a and 12a). By doing so, a negative torque is applied by the flywheel 38 on thepower split device 34, which means that, inversely, a positive torque isapplied by the power split device 34 to the flywheel 38. Thereby,assuming that the flywheel 38 is initially rotating in the positivedirection, it means that the flywheel will accelerate and indeed storesome additional energy recovered from the main driveline. As can beseen, the energy recovery system operates at the same time mechanicallyand electrically and the recovered energy is thus transferred and storedthrough two forms, both mechanically and electrically.

The balance between the recovered electric energy and the electricenergy needed to supply both the power split device and the electricalnetwork can be tuned by choosing appropriate planetary gear ratios. Thisenergy balance can be further tuned by the gearshift strategy. Forinstance, in case of an automatic or automated gearbox, speeds upstreamof the gearbox can be increased during braking and lowered duringvehicle take-off in order to maximize electric energy regeneration bythe first electric motor.

It is to be noted that, when a resistive torque is needed on the secondinput/output 40 of the power split device 34 (i.e. the ring wheel 41 inour example), the first electrical machine 42 can be either supplementedor replaced by the brake 46 to provide such torque. This may bedesirable in various operating cases. It may for instance be of interestwhen the electrical machine 42 would be used as a generator, for examplein the case where the batteries 52 are full so that it is not possibleanymore to store electricity. In any case, the presence of the brake 46can help in reducing the electrical machine's rating because at leastpart of the resistive torque can be provided by the brake 46, which ofcourse may lead to using a cheaper, lighter and less bulky electricalmachine and electric energy storing system.

The extra torque supplied by the brake can enable to start the ICEeither when the vehicle is at stand still or when the vehicle is runningin flywheel mode. When starting the ICE when the vehicle is at standstill, the first clutch can be closed without energy loss (the carrierwill be stopped as long as no torque is applied on the brake or thefirst electric motor). Applying torque with the brake will start theICE. The energy loss in the brake is compensated by avoiding the energyloss that usually occurs in the first clutch when connecting the ICE tothe gearbox. When starting the ICE and synchronizing its speed to thatof the gearbox input, when vehicle is running in flywheel mode, theadditional torque supplied by the brake avoids torque loss on the drivenunit. This improves the driving comfort. Moreover, the brake is adaptedto start up the flywheel when it is discharged. In such a case, all theenergy coming from the drive train and/or from the engine can betransferred to the flywheel, because, the second input/output of thepower split device being blocked by the brake, no power is send to thatinput/output.

To get the energy from the flywheel 38, the latter must be deceleratedby applying a resistant torque on its shaft, whereby inversely, theflywheel 38 will apply a motoring torque on the power split device'sfirst input/output 36. As above, this can be achieved by proper controlof the first electrical machine 42 (and/or of the brake 46 if present).

Such a case is shown on the simplified functional diagrams of FIGS. 13 band 14 b (which correspond respectively to the speed diagrams of FIGS.13 a and 14 a) where it is represented by an arrow that the flywheel 38is able to contribute power to the power split device 34. This torque isthe counterpart of the flywheel deceleration. We use the conventionaccording to which the torque applied by the flywheel 38 to the powersplit device 34 in such a case is positive, represented by an upwardarrow on the speed diagrams. Of course, the aim is to have at least partof the energy provided by the flywheel 38 be sent to the vehicle wheels.Therefore, as can be seen on FIG. 13 a, the first electrical machine 42needs to be controlled to exert a certain torque on the power splitdevice 34. Depending on the rotation direction of the electric machine(opposite cases are shown on FIGS. 13 a, 14 a and FIGS. 13 b, 14 b), theelectrical machine 42 will be controlled either as generator (whereby itextracts mechanical power from the power split device 34, as shown onFIG. 14 b, this power being transformed into electricity and provided tothe electrical network 50), or as a motor (whereby it providesmechanical power to the power split device 34, as shown on FIG. 13 b,this power being provided by the electrical network 50) to, in bothcases apply, on the power split device 34 a torque which, according tothe above convention, is positive. By doing so, a negative torque isapplied by the main driveline on the power split device 34, which meansthat, inversely, a positive torque is applied by the power split device34 to the flywheel 38. Thereby, assuming that the main driveline isinitially rotating in the positive direction, it means that the maindriveline will tend to accelerate and indeed benefit from some recoveredenergy. As can be seen again, the energy recovery system operates at thesame time mechanically and electrically and the recovered energy is thustransferred through two forms, both mechanically and electrically.

On FIG. 2 is shown a second embodiment of the invention, which isvirtually identical to the first embodiment, except for an additionalelectric machine 58 which is mechanically coupled to the engine outputshaft 14 (or in other words with the gearbox input shaft 16) through amechanical transmission with a constant ratio (not shown). This secondelectrical machine 58 is essentially a traction motor which forms,together with the engine 12, a hybrid parallel traction power unit whereboth the engine and the motor are able to drive the vehicle, eithertogether or independently. The second electrical machine 58 iselectrically coupled to the electrical network 50, so that it can drawor provide electrical current from/to the network. Therefore, the secondelectrical machine 50 can draw current from the electrical storage unit52 or from the first electrical unit 42 when the latter is used as agenerator. Conversely, the second electrical machine 58 can also beoperated as a generator and provide current to be stored in theelectrical storage unit 52 or to be used in the first electrical machine42 when it operates as a motor.

The second electrical machine allows a transfer of energy between theflywheel and the batteries in order to manage the state of charge (SOC)of both of them independently. In addition, the second electric machine58 can assist in supplying the first electric machine 42 in certainoperating conditions where it is possible to mechanically draw energyfrom the main driveline, so that the size of the batteries can beoptimized. Also, the second electric machine 58 can be rated in order toprovide most of the energy to operate the first electrical machine 42,so that the size of the batteries can be minimized. To the extreme, thebatteries could simply be omitted.

In the case of a powertrain for driving a medium sized truck, such asecond electrical machine could have a power rating of approximately20-80 kWh.

On FIG. 3 is shown a third embodiment of the invention which is verysimilar to the second embodiment, except that a third electrical machine60 is provided (in place of the second electrical machine of embodimentT) which is mechanically coupled to the flywheel 38 (or in other wordswith the first input/output of the power split device 34) through amechanical transmission with a constant ratio. This third electricalmachine 60 is of course electrically connected to the electrical network50, so that it can draw or provide electrical current from/to thenetwork, depending on whether it is operated as a motor or a generator.Therefore, the third electrical machine 60 can draw current from theelectrical storage unit 52 (or from the first electrical unit 42 whenthe latter is used as a generator) to speed up the flywheel 38.Conversely, the third electrical machine can also be operated as agenerator and provide current to be stored in the electrical storageunit 52 when it operates as a motor drawing its energy from the flywheel38.

As in the second embodiment, the third electric machine 60 can be usedto transfer energy from one storage means to the other, in order tomanage their state of charge independently.

The third electric machine 60 can also assist the batteries to operatethe first electrical machine by using an amount of energy from theflywheel 38. At last, the third electric machine 60 can be rated inorder to provide most of the energy to operate the first electricalmachine 42, so that the size of the batteries can be minimized.

The fourth embodiment shown on FIG. 4 is simply a combination of thesecond and third embodiments, with both the second 58 and the third 60electrical machines as described above. This embodiment has theadvantage of allowing an optimum and flexible management of the energyrecovery, optimizing the use and storage of the energy. This embodimentenables to transfer part of the energy in electric mode to and from theflywheel.

In all the cases above, it is optimum to have the power split device 34which is coupled to the main driveline “upstream” of the variable ratiotransmission device 18. Indeed, such a transmission device 18 is used tomultiply the torque available for the driven unit for a given poweroutput of the engine, simply by decreasing the rotation speed.Therefore, in general, the torque values which will transit through thepower split device 34 will be lower if the latter is coupled “upstream”of the gearbox rather than downstream. This in itself allows usingsmaller, lighter and cheaper components for the power split device.

Also, the power split device being coupled upstream of the gearbox 18,its second input/output 40 is maintained with a speed range which is nottoo extended. Indeed, if the engine is a diesel engine for a commercialtruck, the engine will operate approximately between 600 and 2400 rpms,which represents a factor of 4 between the minimum and the maximumspeed. Downstream of the gearbox 18, such a factor would be muchgreater. From there, it must be taken into consideration that theelectrical machine must have an even wider range of operation, becauseit must be varied as function of the speed of the main driveline and ofthe speed of the flywheel 38 according to the speed diagrams to achievethe power split effect. Therefore, the first electrical machine 42 hasto be able to operate under wide ranges and it is of course of interestto minimize that operating range so that a less expensive machine can beused. A major benefit is to operate the electric machine near its pointsof best efficiency.

FIGS. 5 to 9 show very schematically how the energy recovery system canbe structurally integrated in the powertrain according to the invention.On those figures, clutches 20, 22 are omitted.

On FIG. 5 is shown one possible structural embodiment of the firstfunctional embodiment of FIG. 1. One can see that the planetary gear 34,the flywheel 38 and the first electrical machine 42 of the energyrecovery system are all mounted coaxially on the engine output shaft 14,and can therefore be comprised within a common housing 62 which can be amere enlargement of the conventional engine flywheel and clutch housingprovided on conventional piston engines. Thus, the energy recoverysystem can be very tightly packed, without changing the general layouton the powertrain compared to a conventional vehicle. The onlymodification is basically and slight increase of the engine lengths byapproximately 15 to 30 centimeters. A part from that, the only bulkyadditional element is the electrical storage means, which can be locatedanywhere on the vehicle. In this embodiment, it appears that theflywheel 38 is coupled to the sun wheel 37 of the planetary gear throughan auxiliary planetary wheel 64, the engine output shaft 14 is coupleddirectly to the satellite carrier 43 and the first electrical machine 42is directly coupled to the ring wheel 41. In fact, in this firstembodiment, the ring wheel 41 is unitary with the first electricalmachine's rotor, or at least affixed to it.

On FIG. 6 is shown another possible structural embodiment of the firstfunctional embodiment of FIG. 1. Compared to that of FIG. 6, theflywheel, the planetary gear and the auxiliary planetary gear 64 havesimply been reversed axially, with the flywheel 38 on the engine side ofthe housing 62 rather than on the gearbox side of the housing. Moreimportantly, the first electrical machine 42 is not anymore integratedin the housing 62. It is now set in parallel to the flywheel axis, andit is coupled through a gearing mechanism 66 to the ring wheel 41 of thepower split device 34. This layout allows using a first electricalmachine 42 having a smaller diameter.

On FIG. 7 is shown one possible structural embodiment of the secondfunctional embodiment of FIG. 2. This embodiment is directly derivedfrom that of FIG. 5, where the second electrical machine 58 is alsointegrated in the same housing 62. This second electrical machine'srotor is directly coupled to the engine output shaft. In the figure, thesecond electrical is shown on the engine side of the housing, betweenthe engine and the recovery system, but it could also be located on theother side of the energy.

On FIG. 8 is shown another possible structural embodiment of the secondfunctional embodiment of FIG. 2, showing the same difference with theembodiment of FIG. 7 than the difference between the embodiments ofFIGS. 6 and 5, that is with a non-coaxial externally located smalldiameter first electrical machine 42 which is coupled to the planetarygear's ring wheel 41 through a gearing 66.

On FIG. 9 is shown one possible structural embodiment of the thirdfunctional embodiment of FIG. 3. This embodiment is directly derivedfrom that of FIG. 5, except that it can be seen that the thirdelectrical machine 60 is coupled together with the flywheel 38 on thesame shaft of the planetary gear 64, the second electrical machine beingabsent.

FIGS. 15 and 16 show further alternate embodiments of the secondfunctional embodiment of FIG. 2 where, instead of having the power splitdevice integrated in the flywheel housing, it is located outside of thathousing, on a parallel axis. In both of theses embodiments, the powersplit device 34 has its ring gear 41 which is directly coupled to therotor of the first electrical machine 42, which is here a small diametermachine as in the embodiment of FIG. 8. The power split device 34 andthe first electrical machine 42 are here coaxial. The carrier 43 of thepower split device is mechanically coupled to the flywheel 38 throughmeshed gears 68 (which in this case are integrated in the flywheelhousing 62), and the sun wheel 37 is coupled with the output shaft 14 ofthe engine 12 through another set of meshed gears 70 (which is in thiscase located outside of the flywheel housing).

In the embodiment of FIG. 15, the second electrical machine 58 iscoaxial with the output shaft 14 to which its rotor is directly coupled,and it is located within the flywheel housing 62.

In the embodiment of FIG. 16, the second electrical machine 58 islocated outside of the flywheel housing. It is in fact a small diametermachine, coaxial with the first electrical machine and with the powersplit device. It's rotor is directly coupled to a shaft which connectsthe sun wheel 37 to the set of meshed gear 70, so that the secondelectrical machine is mechanically coupled to the output shaft 14.

In FIG. 17 is shown an alternate structural embodiment of the thirdfunctional embodiment of FIG. 3. This embodiment has the same layout asthe embodiment of FIG. 15 with respect to the power split device and thefirst electrical machine. It does not have a second electrical machine(although it could well be provided), but a third electrical machine 60.The third electrical machine 60 is a small diameter machine, external tothe flywheel housing 62, and not coaxial neither with the output shaft14 nor with the power split device 34. In fact, the third electricalmachine is also coupled to the flywheel 38 through a set of meshedgearings.

1. Powertrain comprising: a variable ratio transmission device having aninput shaft coupled to an engine and an output shaft coupled to a drivenunit; a three-way power split transmission device comprising threeinput/output couplings, a first of which is coupled to a flywheel and asecond of which is coupled to an electrical machine; and a brake coupledto the second input/output coupling of the three-way power splittransmission device, in parallel to or in series with the first electricmachine; wherein the third input/output coupling of the three way powersplit transmission device is mechanically coupled to the engine and tothe input shaft of the variable ratio transmission device.
 2. Powertrainaccording to claim 1, wherein the power split transmission device is aplanetary gear comprising a sun wheel, a ring wheel coaxial with the sunwheel, and a carrier which is coaxial with the sun and ring wheels,where the sun wheel, the ring wheel and the carrier are rotatable one tothe other around their common axis, and where the carrier carriesrotatable satellite wheels which are meshed with both the sun wheel andthe ring wheel.
 3. Powertrain according to claim 1, wherein the flywheelis coupled to the first input of the power split transmission devicethrough a reduction gear.
 4. Powertrain according to claim 3, whereinthe reduction gear comprises a further planetary gear.
 5. Powertrainaccording to claim 1, wherein the electrical machine is electricallyconnected to electrical auxiliaries and/or to electrical storage meansand/or to an electric resistor.
 6. Powertrain according to claim 1,comprising a second electrical machine which is mechanically coupled tothe driven unit.
 7. Powertrain comprising: a variable ratio transmissiondevice having an input shaft coupled to an engine and an output shaftcoupled to a driven unit; a three-way power split transmission devicecomprising three input/output couplings, a first of which is coupled toa flywheel and a second of which is coupled to an electrical machine;and a second electrical machine which is mechanically coupled to thedriven unit, wherein the third input/output coupling of the three waypower split transmission device is mechanically coupled to the engineand to the input shaft of the variable ratio transmission device,wherein the second electrical machine is mechanically coupled to theinput of the variable ratio transmission device.
 8. Powertrain accordingto claim 1, wherein the variable ratio transmission device is a discreteratio gearbox.
 9. Powertrain according to claim 8, wherein the discreteratio gearbox is a mechanical or automatic gearbox.
 10. Powertrainaccording to claim 8, wherein the discrete ratio gearbox is an automatedmechanical gearbox.
 11. Powertrain according to claim 1, wherein theengine is coupled to the input of the variable ratio transmission devicethrough at least one clutch mechanism.
 12. Powertrain according to claim6, comprising a third electrical machine which is mechanically coupledto the flywheel.
 13. Powertrain according to claim 1, wherein theflywheel and the three-way power split transmission device areintegrated coaxially in a housing fixed on the engine.
 14. Powertrainaccording to claim 13, wherein the first electrical machine is mountedcoaxially in the housing.
 15. Powertrain according to claim 13, whereinthe first electrical machine is mounted in parallel to the flywheelaxis.
 16. Powertrain according to claim 6, wherein the flywheel and thethree-way power split transmission device are integrated coaxially in ahousing fixed on the engine, and the second electrical machine ismounted coaxially in the housing.
 17. Powertrain according to claim 2,wherein the flywheel is coupled to the sun wheel of the planetary wheel,the engine is coupled to the satellite carrier and the electricalmachine is coupled to the ring wheel.